The invention relates to a fluid friction coupling which has a reservoir chamber and a working chamber separated by a partition such as is known, for example, from DE-C 28 14 608.
In known couplings of this type, the technical problem often is the fact that the idling rotational speed, i.e., the rotational speed at which the driven part of the coupling rotates with the coupling switched off, i.e., with a closed valve lever, is too low. At such low idling rotational speeds, the coupling does not switch on when required because the pressure in the reservoir chamber is too low.
In order to increase the idling rotational speed, it has already been proposed that so-called leakage oil holes should be provided in the partition between the reservoir chamber and the working chamber. There is a continuous flow of oil through these leakage oil holes from the reservoir chamber to the working chamber so that the degree of filling of the working chamber is increased and with it, simultaneously, the idling rotational speed (DE-GM 83 23 498). The leakage oil holes are not, however, sufficient for all purposes.